1. Field of the Invention
The present invention relates to a waveguide for the efficient detection and collection of back-scattered electrons in electron beam apparatuses such as electron microscopes.
2. Description of the Related Art
Back-scattered electrons are detected using scintillators. Typically electrons are directed to strike a phosphor coating on a piece of glass or plastic so that the light generated by the phosphor is directed onto a detector placed at one end of a light guide. A typical light guide is shown in FIG. 1. The drawback with this type of detector is that it has a low collection efficiency, thus limiting the speed at which the system can be operated. Also, the collection efficiency is not very homogeneous throughout the phosphor coating, i.e. the collection efficiency is a strong function of where the electron strikes the phosphor. Electrons that strike the phosphor on the same side of the hole as the detector are much more efficiently collected than those that strike the opposite side of the hole from the detector. The collection inhomogeneity leads to a reduced contrast depending on how the electrons scatter from the sample.
A primary object of this invention is to provide a more efficient waveguide for detecting back-scattered and secondary electrons, particularly back-scattered or secondary electrons reflected from or emitted by a sample scanned by an electron beam from a minicolumn or microcolumn used in an electron microscope. A waveguide of optical glass is designed so that, when back-scattered or secondary electrons are detected by a phosphor deposited on the waveguide, the light emitted by the phosphor is reflected internally along the waveguide to a detector. The inventive design includes surfaces in the region of the phosphor deposition which are coated with a highly reflective material to direct the light towards the end of the waveguide to which the detector is coupled. Further, the waveguide may include a flared section at the detector end to ensure that all rays striking the output face of the waveguide strike the surface at angles less than the critical angle and are transmitted to the detector.